1. Field of the Invention
The present invention relates to a transformer, and more particularly to a transmission line transformer which is capable of improving efficiency and a dynamic range of a power amplifier, and which is formed to have a plurality of load impedances as the primary side of the transmission line transformer is separated to form a plurality of primary transmission lines with parasitic components which are different from each other, in which the transmission line transformer is used as an impedance matching circuit of the power amplifier requiring a plurality of load impedances.
2. Description of the Related Art
Generally, a power amplifier requiring a plurality of load impedances is usually used in a wireless communication system.
FIG. 1 is a schematic block diagram of a power amplifier of a general art, or a power amplifier with a polar structure which is a type of wireless communication system.
In general, a power amplifier with a Class E output stage structure which is used in a polar structure guarantees its dynamic range as its source voltage VDD is adjusted. The output power of the power amplifier can be expressed as following Equation (1):
                              P          out                ⁢        ∞        ⁢                              VDD            2                                R            load                                              (        1        )            
Where Pout denotes output power, VDD denotes a source voltage, and Rload denotes a load impedance.
As described in Equation (1), when the source voltage VDD is controlled, the output power Pout is changed. However, the conventional power amplifier has disadvantages in that its sufficient dynamic range cannot be guaranteed by only control of the source voltage VDD.
FIG. 2 is a graph describing a dynamic range based on a source voltage of a non-linear power amplifier of a general art, in which the dynamic range can be extended as the load impedance is controlled.
For example, as described in Equation (1), the Class E output stage of the conventional determines its dynamic range according to change of the source voltage VDD. Namely, when the source voltage is changed from 0.6V to 3.3V, the dynamic range of the power amplifier is 16.4 dB based on Equation (1).
However, the wireless communication system requires a relatively wide dynamic range. Therefore, to comply with such a request, when the source voltage VDD is less than a source voltage VDD of point 203 which is a point where load impedance Rload is increased, if load impedance Rload is increased, the dynamic range is extended as reference numerals 201 and 202 indicated.
As described in Equation (1), a dynamic range is generally extended as load impedance is increased at less than a predetermined source voltage VDD.
Also, the greater the load impedance in a low output power region, the more efficiency of the power amplifier is increased. Namely, when the load impedance value is increased in the low output power region, the dynamic range and efficiency of the power amplifier are increased.
However, since load impedance Rload of the conventional power amplifier is set to a proper value to meet with the maximum value of the output power Pout, the power amplifier decreases its efficiency in a region where a relatively low output power is outputted.
Therefore, a plurality of load impedances is installed in the power amplifier in order to resolve the problems.
Of the methods where the power amplifier has a plurality of load impedances installed, there is a method wherein the final stage of the power amplifier is separated into plural parts. Namely, the plural parts are divided into a high output power part, a middle output power part and a low output power part.
Here, the high, middle, and low output power parts are configured with impedance matching circuits which have load impedances complying with high, middle, and low output powers, respectively.
Therefore, the amplifier is operated such that: when requiring high output power, all of the three parts are turned on; when requiring middle output power, the high output power part is turned off and the other parts are tuned on; and when requiring low output power, the high and middle output power parts are turned off and the low output power part is turned on.
As such, since the part for outputting low output is configured with an impedance matching circuit with load impedances to comply with low power, efficiency of the power amplifier is increased. Therefore, the power amplifier has characteristics that its efficiency is high in the whole range of output power.
Such a method was disclosed in a journal, “A. Shirvani, et al., ‘A CMOS RF Power Amplifier With Parallel Amplification for Efficient Power Control,’ IEEE J. Solid-State Circuits, vol. 37, no. 6, pp. 684-693, June 2002.”
In order to output the respective output powers, an amplification stage of a power amplifier of a general art can be divided into a low output power part and a high output power part, as shown in FIG. 3. Namely, the low and high output power parts are configured with low and high output power impedance matching circuits each of which has different load impedance.
Namely, the parts for outputting high and low powers are configured with load impedances to comply with high and low powers, respectively. Therefore, the part for outputting high power can be turned off if the power amplifier outputs low output power.
As such, the amplification stage of the power amplifier is divided into two parts to have two load impedances, configured as two impedance matching circuits. Here, reference numerals 303 and 304 denote amplifiers for high and low powers in the power amplifier, respectively. The respective amplifiers are connected to the impedance matching circuit 302. Namely, the amplifiers for high and low powers are connected to the high and low output power impedance matching circuits, respectively.
Therefore, the conventional power amplifier can improve its efficiency and dynamic range as it is operated such that: the amplifier 303 for high power and the high output power impedance matching circuit connected thereto are turned on, and the amplifier 304 for low power and the low output power impedance matching circuit connected thereto are turned off, when high power is needed; and the amplifier 303 and the high output power impedance matching circuit connected thereto are turned off, and the amplifier 304 and the low output power impedance matching circuit connected thereto are turned on, when low power is needed.
However, the method disclosed in the above-mentioned journal has disadvantages in that it requires additional switches such that another turned on amplification stage can have a desired load impedance when a part of the amplification stage is turned off, and also its circuit size can be increased since the size of the switches is similar to that of a power transistor in the amplification stage.
Also, the conventional power amplifier has drawbacks in that, since it adopts a structure of Class F output stage (a type of Class F power amplifier), the size of its entire circuit can be decreased only if the operation frequency is greater than tens of gigahertz.